[vlanmgr] Support Jumbo Frame By Default (sonic-net#1393)

The bridge created for vlans has a a default MTU of 1500 which is
serving as default for vlans added to the bridge. This PR changes
the default MTU to 9100 to be inline with portchannels and other
interfaces.

signed-off-by: Tamer Ahmed <tamer.ahmed@microsoft.com>

cfgmgr/vlanmgr.cpp

@@ -51,15 +51,17 @@ VlanMgr::VlanMgr(DBConnector *cfgDb, DBConnector *appDb, DBConnector *stateDb, c
     // The command should be generated as:
     // /bin/bash -c "/sbin/ip link del Bridge 2>/dev/null ;
     //               /sbin/ip link add Bridge up type bridge &&
+    //               /sbin/ip link set Bridge mtu {{ mtu_size }} &&
     //               /sbin/bridge vlan del vid 1 dev Bridge self;
     //               /sbin/ip link del dummy 2>/dev/null;
     //               /sbin/ip link add dummy type dummy &&
-    //               sbin/ip link set dummy master Bridge"
+    //               /sbin/ip link set dummy master Bridge"
 
     const std::string cmds = std::string("")
       + BASH_CMD + " -c \""
       + IP_CMD + " link del " + DOT1Q_BRIDGE_NAME + " 2>/dev/null; "
       + IP_CMD + " link add " + DOT1Q_BRIDGE_NAME + " up type bridge && "
+      + IP_CMD + " link set " + DOT1Q_BRIDGE_NAME + " mtu " + DEFAULT_MTU_STR + " && "
       + BRIDGE_CMD + " vlan del vid " + DEFAULT_VLAN_ID + " dev " + DOT1Q_BRIDGE_NAME + " self; "
       + IP_CMD + " link del dev dummy 2>/dev/null; "
       + IP_CMD + " link add dummy type dummy && "